1. Field of the Invention
This invention relates to the field of hydrolyzed ethylene-vinyl acetate copolymer and, more particularly, to the field of processes for finely dispersing colorant in hydrolyzed ethylene-vinyl acetate copolymer.
2. Description of the Prior Art
Although there exists a large market for transparent, clear colorless polymers, the use of color in plastic moldings, profile extrusions, sheetings, film and pipe is often highly desirable from the standpoint of consumer appeal and can even be essential for some product applications. For example, plastic housings for most consumer goods are colored for aesthetic reasons. In the case of wire and cable insulation, it is common to incorporate carbon black into a base resin such as polyethylene in order to prevent or retard degradation caused by weathering. Colored resins such as polyvinylchloride are used not only for their attractive appearance and maintenance-free characteristics, the colorants dispersed therein help to protect the resins from the destructive effects of ultraviolet radiation.
Inorganic and some organic based coloring agents are commonly used to color polymeric materials since they can withstand processing temperatures often ranging from about 120.degree. C. to about 350.degree. C., and possess good stability to sunlight, rain and other atmospheric conditions. Colorants such as carbon black, titanium dioxide, phthalocyanine blue, phthalocyanine green, and the like, are generally dispersed within thermoplastic resins by intensively mixing the resins with the requisite amount of colored matter in heated processing equipment such as roller mills, compounding extruders, Banbury mixers, sigma blade mixers, and the like. Both the appearance of the resins and the color values for given amounts of coloring agent can be improved to the extent finer dispersions of colorant particles in the resins are attained. Because many types of colorants including those aforementioned are difficult to disperse in resins, special compounding techniques must often be employed to achieve acceptable dispersions. Specialty processors often offer pigment concentrates and masterbaches to simplify to some extent the process of providing pigmentation with good, speck-free, uniform dispersion characteristics. Regardless of the procedure used, intensive mixers and other massive processing equipment are costly to purchase and in operation consume relatively large quantities of energy. This being the case, coloration costs are generally significantly greater than the mere cost of the colorant material.
Hydrolyzed ethylene-vinyl acetate copolymers constitute an important class of thermoplastic resins known for their excellent stability, good barrier properties and ease of processing. When these copolymers are to be colored, it is the usual practice to incorporate the coloring agent into the resins employing techniques of intensive mixing whose disadvantages have been commented upon above. While it is known that colored particles of ethylene-vinyl acetate copolymer can be prepared by dispersing the copolymer in an aqueous medium containing surface active agent (cf. U.S. Pat. Nos. 3,449,291 and 3,674,736 to Lerman et al. and U.S. Pat. No. 3,669,922 to Bartsch et al.), heretofore no attempt has been made to finely disperse colorant in hydrolyzed ethylene-vinyl acetate copolymer in the course of production of the latter by known and conventional solution hydrolysis procedures such as described in U.S. Pat. Nos. 2,386,347 and 2,399,653 to Roland, Jr., 2,396,210 and 2,434,179 to Sharkey, 2,403,464 to Smith, 2,451,963 to Loder, 3,344,129 to Bestian and 3,386,978 to Salyer.